. HEAT TREATMENT.
Within the last few years there have been radical advances in our knowledge of the structure of steel and the influence exerted by what has come to be known as "heat treatment/' The main principles of this branch of metallurgy have been understood for quite a long time, but they were applied only in exceptional cases, such as the manufacture of guns and armor plate. To-day progressive manufacturers are using the results of research in improving the quality of their ordinary forgings and castings, and it is therefore necessary to consider at some length the general underlying principles of the science of micro-metallography. This has been done in the latter half of this chapter, the article being written by my brother, J. W. Campbell.
The introduction of accurate determinations of temperatures and a better knowledge of the proper heat to use, h#s to a certain extent diminished the value of the experiments and investigations published in the first edition of this book, but I believe they may be worth recording again, as it is quite certain that many non-progressive works will follow the common and ancient methods of annealing both at the forge of the smith and on a larger scale in the treatment of eye bars and similar material. In the following sections the word "annealing" is used unless otherwise stated to signify that the piece was heated in a muffle heated by a soft coal fire, the bar being withdrawn when it had reached a dull yellow heat. The experiments were carefully performed and it is believed that the practice was fairly uniform.
SECTION" XVa.—Effect of annealing on ihe 'physical properties of rolled Jars.—It is a well known fact that annealing tends to remove the strains which are created by cold rolling and distortion, but it is not generally understood how profound are the changes